Interpretive Summary: Recently biochar (charcoal) has been in the news concerning global warming. Biochar is produced by heating woody material in a low oxygen environment which stabilizes the carbon that made up the wood. If added to soil it can reside for a thousand years. The thought is that adding stabilized carbon to soil will improve the soil and reduce CO2 in the atmosphere, where the carbon came from to make the wood. It has been said that biochar is inert which is why it stays in the soil so long. We found that there is a fraction of the biochar that decomposes in soil. The amount of decomposition was directly related to the amount of biochar added to the soil. Thus scientists working with biochar can use this information to design management methods to reduce this decomposition and biochar producers can process wood in different ways to reduce the degradable carbon in the biochar.

Technical Abstract:
The low temperature pyrolysis of organic material produces biochar, a charcoal like substance. Biochar is being promoted as a soil amendment to enhance soil quality, it is also seen as a mechanism of long-term sequestration of carbon. Our experiments tested the hypothesis that biochar is inert in soil. However, we measured an increase in CO2 production from soils after biochar amendment which increased with increasing rates of biochar. The delta-13C signature of the CO2 evolved in the first several days of the incubation was the same as the delta-13C signature of the biochar, confirming that biochar contributed to the CO2 flux. This effect diminished by day 6 of the incubation suggesting that most of the biochar C is slowly decomposing. Thus, aside from this short term mineralization increasing soil C with young biochar may indeed be a long term C storage mechanism.